1. Field of the Invention
The present invention relates to a plant disease resistance-inducing composition, which uses mycelia or fruit bodies of filamentous fungi as a raw material, is environmentally friendly, and is safe to users and consumers, and a process for producing the composition.
2. Background Art
Major conventional disease control methods for agricultural products include a method in which a chemical pesticide such as a fungicide or an invasion inhibiting agent is used, and a method in which a microorganism pesticide such as antagonistic microorganisms or an organism such as a natural enemy organism is used.
In most of chemical pesticides, diseases are controlled by direct action on plant pathogenic fungi. In addition to the chemical pesticides, resistance induction-type pesticides are used in which diseases of crops are controlled by enhancing the disease resistance possessed by plants per se.
The pesticides in most cases exhibit definite action on the plant pathogenic fungi. Continuous use of these pesticides in many cases results in the appearance of mutants resistant to the pesticides. On the other hand, the resistance induction-type pesticides control disease infection by the induction of the resistance of plants rather than the action directly on the plant pathogenic fungi. By virtue of this mechanism, for the resistance induction-type pesticides, up to now, there is no report on the appearance of resistant mutants. Therefore, it is considered that the influence of the resistance induction-type pesticides on environment including other organisms is relatively small.
In agricultural production, the establishment of sustainable agricultural production techniques has been demanded. To meet this demand, the development of environment-conscious agricultural materials is an important task.
Further, an increasing consumer's demand for safer foods in recent years has led to an increase in an ever-increasing demand for organic agricultural products which have been cultivated by taking advantage of the natural cycling function of agriculture. According to the guideline published by The Ministry of Agriculture, Forestry and Fisheries of Japan, materials usable in the production of organic agricultural products are limited to naturally occurring useful mineral materials, plants, animals, and natural substances which have been removed, extracted, or prepared from them. When the use of them as pesticides, e.g., for the control of diseases and insect pests is desired, only pesticides registered based on the Agricultural Chemicals Regulation Law can be used, and antibiotics should not be used.
Thus, control materials usable in the production of organic agricultural products are limited. Therefore, naturally occurring substance-derived highly effective disease control agents as an alternative to the conventional chemical pesticides have been desired. The supply of agricultural materials, which have the above resistance induction-type disease control effects, that is, can prophylactically control diseases, and are derived from naturally occurring materials, could realize pesticides which are safe to agricultural producers as users and consumers and, at the same time, can reduce an environmental burden.
Up to now, pesticides intended for disease resistance induction of plants have been limited to “Oryzemate” (generic name: probenazole, manufactured by Meiji Seika Kaisha Co., Ltd.) and “Bion” (generic name: acibenzolar-S-methyl, manufactured by Novartis) which are registered as rice blast control agents.
Phytoalexin inducers such as decomposition products of polysaccharides (Japanese Patent Laid-Open Publication No. 331016/1993) and uasmonic acid derivatives (Japanese Patent Laid-Open Publication No. 29412/1999) are reported as other disease resistance-inducing substances.
In the above pesticides for disease resistance induction purposes, control effect against various crop diseases is provided by inducing the resistance of plants rather than the action directly on plant pathogenic fungi. Therefore, up to now, there is no report on the appearance of mutants resistant to these pesticides. All of them, however, are chemically synthesized pesticides and thus do not conform to the guideline. Further, it should be noted that, although the above phytoalexin inducers induce the production of phytoalexins which is a disease resistance reaction of plants, there is no clear description on practical disease control effect in the above publications.
Likewise, cerebrosides which have been reported as phytoalexin inducers are known as having control effect on a practically usable level against rice diseases (Japanese Patent No. 2846610 and WO 98/47364). Cerebrosides are a kind of glycosphingolipids and are glycolipids in which hexose is attached to primary alcohol of ceramide through glycoside linkage. Glycosphingolipids are universally present as constituents of membranes in various organism species and have already been used as highly safe cosmetic ingredients. Glycosphingolipids having disease resistance induction activity have been considered as being categorized as cerebrosides (Koga J. et al. (1998) J. Biol. Chem. 48 (27), 31985-31991). Cerebroside B is known to be distributed in Penicillium funiculosum (Kawai G., Ikeda Y., Tubaki K. (1985) Agric. Biol. Chem. 49 (7), 2137-2146), Lentinus edodes (Kawai G (1989) Biochim. Biophys. Acta 1001 (2) 185-190), Pachybasium sp. (Sitrin R. et al. (1988) J. Antibiot. 41 (4), 469-480), and Rhizoctonia sp. (Umemura K. et al. Plant Cell Physiology 41(6), 676-683). Cerebroside B has a structure represented by formula (I):

Cerebroside B is disclosed as a rice disease control agent in WO 98/47364. The control agent disclosed in this patent is one extracted from plant pathogenic fungi. Further, it should be noted that, up to now, there is no report on the disease control effect of cerebroside B and other cerebrosides against agricultural products other than rice.
Regarding the distribution of cerebrosides in Basidiomycetes, it is known that cerebrosides are contained in an ethanol extract of a fruit body of raw Shiitake mushroom (Lentinus edodes) (Kawai G (1989) Biochim. Biophys. Acta 1001(2) 185-190) and cerebrosides can be extracted with acetone from a cultured mycelium of Schizophyllum commune Fr. (Japanese Patent Publication No. 52878/1988). However, the activity of the substances described in the above publications is fruit body inducing activity, and control effect against diseases of plants is not described therein.
Regarding the utilization of extracts of Basidiomycetes in crop cultivation, for example, Japanese Patent Publication No. 44040/1981 describes the utilization of the extracts for increasing the yield of agricultural products or for growth promotion purposes, and Japanese Patent Publication No. 48087/1982 describes the utilization of the extracts for pigmentation promotion and sugar content enhancement in citrus. All of these actions, however, are exhibited by aqueous extracts.
In general, the activity of disease resistance-inducing substances is known to vary depending upon plant species. Glucans, which are polymers of glucose, are known as representative examples of microorganism-derived resistance-inducing substances which have hitherto been reported. Glucans which induce the disease resistance of soybeans do not induce any resistance reaction in rice plants. On the other hand, glucans, which induce the disease resistance of rice plants, do not have any resistance inducing capability in soybeans (Sharp K. (1984) J. Biol. Chem. 259, 11321-11336, and Yamaguchi T. (2000) Plant Cell 12, 817-826), indicating that, despites substances categorized as glucans, the reaction of crop species vary depending upon polymerization methods. Further, arachidonic acids are reported to induce resistance of potatoes (Bostock M. (1981) Science 212, 349-360). However, there is no report on the induction of resistance in crops other than potatoes. Thus, resistance-inducing substances are considered to have crop species-dependent specific activity.